Suppose, just suppose...
I'm not saying I systematically downvote, mind you - because I don't. Still, just for the sake of argument, consider the fact that every user has a finite number of votes.
I tend to use my allotment of votes to upvote good WUs. I like to encourage the good writers, rather than chastise the duffers. But I always reserve a fraction of my votes (call it one or two votes) for downvotes, to be used on especially deserving WUs. When I come across something that is just miserably bad, and not worthy of the time it took me to read it, I try to do my best for the general quality of E2 by downvoting it. I don't indulge in asking the editors to nuke it - the editors decide that. However, if lots of other E2ians feel the same way as me, I feel confident that it will be zapped, sooner or later.
So, how does this tie in with systematic downvoting? Well, it happens that I encounter a particular noder whose talent might be better spent elsewhere (say, on the walls of his high school's bathrooms). Then, having learned that this person is incapable of writing something that lives up to even the lowest standards, I make it a point to look at what else he has done. If his other work is as bad, it gets my daily downvote.
Is this systematic downvoting? Some would call it that. But, trust me, there is no rancour involved. I generally /msg the person in question to advise him what I think is wrong with the WU, before using the downvote. However, characteristically, most of the people to whom this rule applies never reply, nor do they take steps to improve the quality of their WU.
In which case, I say - that is a downvote well spent.